Nanobiosensor
Biosensors are devices, which incorporate a biological element (e.g., enzyme, antibody) to detect chemical, biological and organic species. Biosensors have potential applications including, but not limited to, extreme environments (Dong et al., Electroanalysis, 15, 157, 2003), detection of food and pathogenic bacteria (Ivnitski et al., Electroanalysis, 12, 317, 2000), glucose monitoring (Wang, Electroanalysis 13, 983, 2001) and the food industry.
Conventional sensing electrodes (e.g., gold, platinum, glass, carbon) that have been used to immobilize biological enzymes have shown limited sensitivity and selectivity. In addition, sensor performance has been limited due to possible interfering compounds.
Most electrochemical biosensors operate in the liquid phase. There are instances where the analyte may be in the gaseous form and the electrochemical biosensor would be unable to detect the gaseous species. The performance of the liquid electrolyte sensors are limited by saturation of the analyte in the liquid phase, electrode corrosion and other operation problems including the requirement of continuous stirring to ensure the effective interaction of the analyte with the sensing element.
Aligned carbon nanotubes grown in situ are useful in electrochemical biosensing applications but the difficulties in achieving reproducibility severely limits their use in such applications. High manufacturing costs associated with producing aligned nanotubes further limit their commercialization.
There have been previous reports of electrochemical gas biosensors using ionic conducting films like nafion and tetrabutylammonium toluene-4-sulphonate (TBATS) for the detection of hydrogen peroxide and phenol vapors (Saini et al., Biosensors and Bioelectronics 10, 945, 1995; EP0585113A2), which use specific enzymatic reactions. However, these sensors used an enzyme (horseradish peroxidase) and a mediator (potassium hexacyanoferrate (II)) for sensing hydrogen peroxide with enzyme mediator gels. The “drop and dry” process of the mediator, gel and enzymes did not yield a high sensitive and selective detection. There have been reports about biosensors using a thick film electrochemical device with an insulating substrate for the determination of ethanol vapors using alcohol dehydrogenase enzyme which also involved the “drop and dry” process (EP634488A2).
CNT-TFT
Chemical sensors are devices that detect chemical and biological species based upon an interaction between two molecules. These sensors can be used to detect various analytes in gas, liquid and solid phases. Sensors can be manufactured to operate in ambient or extreme environmental conditions. When optimized, chemical sensors can detect very low levels of a desired analyte, however, the drawback is usually the large amount of support equipment. This equipment usually prevents the sensors from being portable.
Conventional sensors can be made using a wide variety of techniques, each specific to the desired detectable analyte. A suitable technique would focus on the interactions between two molecules that result in signal generation. The signal produced could be light emission, electron transfer or other physical change. Every sensor needs a method of transduction, i.e., converting the chemical event to a measurable output signal.
Current methods of detection have limited selectivity when operating at the limits of detection. At these extremes of performance, separating a signal from the surrounding noise becomes extremely difficult. One method for increasing this signal to noise ratio is to have internal amplification of the desired signal. Internal amplification prevents additional noise being introduced into a detection system through outside electronics. A simple way to achieve amplification is to build the detector using transistor architecture. This architecture can take advantage of the inherent gain associated with a semiconducting material.
Carbon nanotube (CNT) transistors have been known for several years. (Tan et al., Nature, 1998 (393) 49, Martel et al., Appl. Phys. Lett. 1998 (73) 2447). Many examples of these devices rely on a single CNT placed between electrodes. These devices are difficult to prepare, requiring tedious placement of electrodes with respect to CNT position. These are advanced techniques that require highly specialized instrumentation including electron microscopes and electron beam writing. The use of this instrumentation required for characterization and fabrication prevents this from being a manufacturable technique.